As a normal cell develops into a solid tumor it undergoes a series of changes. At the genetic level, oncogenes are activated and multiple tumor suppressor genes are inactivated. At the physiological level, growth is enhanced, immunity evaded, and neovascularization induced. Neovascularization appears to be a prerequisite. Experimental solid tumors are unable to grow beyond a few millimeters in thickness without a blood supply. Most natural solid tumors elaborate angiogenic factors that attract the new vessels on which they depend. [For a discussion of angiogenic factors and the unsolved problem of how to inhibit tumor neovascularization, see Folkman and Klagsburn, Science, 235:442-447 (1987).] It has become increasingly evident that once a solid tumor has been established in the body every increase in tumor cell population must be preceded by an increase in new capillaries that converge upon the tumor. Consequently, there has been a continuing research effort directed toward the question of what prevents rampant capillary proliferation and what maintains the quiescent state of the capillary endothelial cells of normal tissues.
There has also been an active search for a therapeutic agent or agents which can cause capillary regression. Identification of such an agent has proven to be a very difficult problem. About the only demonstrable difference between tumor angiogenesis and other types of non-neoplastic angiogenesis is a greater intensity and persistence of the angiogenesis induced by tumors. It has become generally recognized that a therapeutic agent which can effectively inhibit tumor neovascularization should be of great value in limiting, or even completely stopping, the growth of tumors.
In one investigation of angiogenesis, Bouck, et al. carried out tests with a panel of cell hybrids derived from fusions between a chemically transformed hamster cell line and normal human fibroblasts [Cancer Res. 46:5101-5105 (1986)]. These researchers reported that anchorage independence of the cells (which in these cells is 100% correlated with tumor forming ability) is initially suppressed and that to remain repressed they must retain human chromosome 1. These researchers further found that the suppressed hybrids were unable to elicit an angiogenic response in a rat cornea assay. In contrast, those hybrids on which anchorage independence is expressed and which have lost human chromosome 1 were found to be potently angiogenic.
Dr. Noel Bouck, Dr. Peter J. Polverini and their associates in the Departments of Microbiology Immunology and Pathology of Northwestern University Medical School reported on further work with their hamster cell lines [Rastinejad, et al., Proceedings, 78th Ann. Meeting, Amer. Assoc. Cancer Res., Vol. 228:61, Abstract 241 (March, 1987)]. Exploration of the possibility that phenotypes of anchorage independence and angiogenesis depend on a common mediator, utilized the transforming growth factor (TGF-B). In testing for angiogenic activity, mixing experiments were performed to investigate the lack of angiogenic response to a mixture of normal baby hamster kidney (BHK) cells and a transformed cell line producing TGF-B. It was found that the normal BHK cells or their conditioned media inhibited angiogenesis when co-introduced with transformed cells into the cornea. It was concluded that secreted factors in the non-tumorogenic lines can negate the angiogenic response to TGF-B.
In 1988 these same investigators (Rastinejad, et al.) reported on the finding of an inhibitor of angiogenesis under control of a cancer suppressor gene [Proceedings, 79th Meeting, Amer. Assoc. Cancer Res., 29:45, Abstract 1809 (March, 1988)]. It had been reported that in a normal baby hamster kidney (BHK) cell line, inactivation of a suppressor gene by carbinogen treatment permitted the expression of anchorage independence and tumorgenicity [Bouck and Head, In Vitro Cell and Devel. Biol. 21:463 (1985)]. Significantly, this loss of suppressor gene function coincided with loss of the ability to elaborate a factor that inhibits neovascularization in the right rat cornea, but anchorage dependent revertants were unable to suppress transformation infusions and failed to produce the inhibitor. Using the ability of this unknown factor to inhibit bovine endothelial cell migration in a modified Boyden chamber, the factor was purified to apparent homogeneity. These results suggested that one function of the cancer suppressor gene present in the normal BHK cells is to mediate the release of an inhibitor of the antiogenesis that is vital to progressive growth of tumors.